The invention relates to the treatment of surfaces.
The treatment of bearing surfaces to provide surfaces with improved wear resistance is well known. It is also well known to lap metal surfaces making use of an abrasive powder composed of hard grains or grit particles, such as silicon carbide particles, to obtain the exact finished dimensions required. That is in lapping some material is removed to obtain those required dimensions. Further, it is well known that wear properties can be improved if such particles are embedded into surfaces. In some methods the impregnation of such particles into the surface is combined with the lapping. Details of the application of this combined technique to cylindrical bearing surfaces are provided in our prior Patent No. GB 1,372,171, and in respect of cylindrical and swept bearing surfaces in our prior Patent No. GB 1,574,141.
However, such methods described in the prior art generally, and in these earlier patents, suffer from a number of disadvantages, for example:
(a) large mounts of the abrasive powder and carrying media are wasted during the production process;
(b) the tooling used in the embedding process is in essence a consumable adding further to process cost, and
(c) the process is difficult to automate due to a high level of manual intervention.
Many of the prior art methods for treating bearing surfaces also suffer from the disadvantage that in the course of treatment base material of the surface is cut or ground away thus dimensionally altering the component rather than just altering the properties of the surface. Such methods are for examples described in earlier patents assigned to the present assignee U.S. Pat. No. 3,171,189 and U.S. Pat. No. 3,961,104, and of North American Rockwell Corporation, U.S. Pat. No. 3,670,462 and of ING Walzlager Schaeffler KG, U.S. Pat. No. 4,567,695.
It is an object of the present invention to provide a new method of treating surfaces to provide improve wear resistance.
According to the present invention there is provided a method of treating a surface of a component formed from a base material, the method comprising the steps of bringing the surface into proximity with free running rollers, moving the surface relative to the free running rollers with a contact force in the range 5.25 to 20.00 Newtons per mm of roller width, and introducing particles of a higher relative hardness than the base material between the surface and the free running rollers, such that at least some of the particles are forced into the base material of the surface by the free running rollers without substantially dimensionally altering the component.
The invention provides a number of advantages. First, durable tooling is used with considerably longer life than the type used on the previous methods such as lapping, thus providing significant cost savings. Second, considerably less carrying media is used, thus providing a further reduction in processing cost but it also leading to reduced post process cleaning after treatment, providing yet further reduction in cost. Third, the force applied to the free running rollers introduces a degree of residual stress into the base material surface and this enhances hard particle retention. Fourth, highly localised areas may be treated on a surface providing improved wear characteristics only where required; eliminating the need to treat areas not required provides further cost savings. Fifth, the process may be readily automated and used in production line manufacturing, with obvious advantages. Sixth, relatively soft base materials, such as aluminium and aluminium alloys, can be treated without scoring or damaging the surface which is a problem when using the previous methods. Finally, non cylindrical surfaces may be treated, giving a much wider range of applications for this method.
Preferably the method also introduces a degree of residual stress into the base material of the surface.
The particles which are introduced, which for convenience will be described as xe2x80x9chard particlesxe2x80x9d, may be introduced between the rollers and the surface in a slurry applied to the surface. Alternatively, the hard particles may be introduced by adhering them to the surface using a light adhesive.
In a further alternative the hard particles may be adhered to a tape using a light adhesive, and the tape passed around at least one of the free running rollers. The tape may be provided in the form of a cassette.
In yet a further alternative a mask may be manufactured to conform to the surface to be treated and the hard particles adhered to the surface of the mask, the mask then being brought into proximity with the surface, between the surface and the free running rollers.
Preferably the surface is rolled more than once such that the particles are fully embedded in the base material.
The method may comprise the further steps of removing excess hard particles and repeating the movement of the surface relative to the rollers with a contact force in the range 5.25 to 20.00 Newtons per mm of roller width, such that the hard particles previously forced into the surface base material are forced further into the base material and the base material closes around them.
The method may comprise a further step of subsequently using an abrasive to produce the desired surface finish and to remove any loose hard particles from the surface.
Preferably the hard particles are silicon carbide.
The surface may be an internal cylindrical surface with a longitudinal axis and the free running rollers may have longitudinal axes arranged to be parallel with the longitudinal axis of the surface.